The present invention relates to a radiation curable coating for use as an overprint varnish for substrates, and more particularly, to a coating composition which can be applied to cellulosic, polymeric, and metallized substrates, and cured to form a protective film which is thermoformable.
The use of energy curable varnishes and coatings for use on food or beverage containers and various substrates used for food packaging and dinnerware such as plates, bowls, cups, etc. is desirable in order to provide the substrates with properties such as grease and water resistance, gloss, and food release properties. Often, such coatings are provided as an overprint varnish applied to the outer surface of the packaging or substrate and over any graphics printed on the packaging or substrate.
Typically, such coatings comprise radiation-curable acrylate resins. However, polymerization of acrylates by ultraviolet radiation typically requires the addition of a photoinitiator. As traditional ketone-type photoinitiators are capable of producing hazardous products upon irradiation, the U.S. Food and Drug Administration has prohibited the use of most photoinitiators in food packaging applications where the coatings are in direct contact with food. Even in applications where the coatings do not directly contact the packaged food product, there is still a concern that one or more components of the coating may migrate through the packaging film/substrate to contact the packaged food. Thus, the use of energy-curable coating compositions for food packaging applications has been limited.
More recently, the United States Food and Drug Administration has cleared for use in food contact applications a mixture of one or more of tripropylene glycol diacrylate (TPGDA), trimethylolpropane triacrylate (TMPTA), trimethylolpropane ethoxylate triacrylate (TMPEOTA) and bisphenol A diglycidyl ether diacrylate, and an optional difunctional alpha-hydroxy ketone photoinitiator, cured by either ultraviolet or electron beam radiation, for use in food contact applications. See Food Contact Notification (FCN 772, Mar. 7, 2008).
Radiation-curable resins are also known in the art which include a “built-in” photoinitiator such that problems with odor which occur with the use of photoinitiators are eliminated. See, for example, US 2008/0039543, US 2008/0085981, and U.S. Pat. No. 7,317,061.
Even with these improvements, it has been difficult to develop energy curable coatings suitable for use on thermoformable food-carrying or food-containing substrates, i.e., substrates formed by heating the substrate to a temperature where it can be molded to a desired shape, such as cellulosic or polymeric plates, cups, bowls, containers, and the like. For example, pre-cured urethane acrylates, which are typically used in overprint varnishes and coatings, have high molecular weights and high viscosities which render them unsuitable for use as thermoformable coatings. While monomers can be added to lower the viscosity of such coatings during application, their flexibility is consequently reduced when cured due to increased cross-link density, which in turn, prevents or adversely affects the thermoformability of the coatings.
It would be desirable to be able to provide an energy curable composition for coating on substrates and packaging which provides desired food release, gloss, and grease and water resistance properties. It would also be desirable for such a coating to be thermoformable after curing.
Accordingly, there is a need in the art for an energy curable composition which can be applied to substrates and cured to form a protective film or coating which can be subjected to thermoforming when the substrates are subsequently heated and shaped into food packaging, containers, or dinnerware.